Multiple myeloma (MM) is a cancer characterized by colonial proliferation of plasma cells in bone marrow, lytic bone lesions, and serum monoclonal gammopathy. MM is the 2th most frequent hematological cancer in the US. The median survival span of MM patients is only 2~6 years, despite treatment either with conventional chemotherapy with or without high-dose therapy/autologous stem cell transplantation, or with new drugs which have novel mechanisms. Increasingly evidence shows that a super family of small (20~24 nt) non-coding RNAs named microRNAs (miRNAs) are involved in cell differentiation, proliferation and apoptosis by targeting 3'UTR of mRNAs of protein coding genes, and therefore involved in cancer pathogenesis, including MM. In our previous work, we identified that two miRNA super families were most significantly deregulated in human MM and cancer stem cells (CSCs) compartment: down-regulated miR-30s, and up-regulated miR-222-221. Moreover, miR-222-221 are the most up-regulated miRNAs in MM. Based on our preliminary data, over- expression of miR-222-221 targets PUMA leads to dexamethasone resistance and CSC accumulation in MM. Nevertheless, it remains unexplored whether miR-222-221 will initiate MM and promote malignant progression in vivo. Accumulating evidence also implicated that miR-222-221 plays a role as oncogene in vitro in a large variety of other cancers, but a causal role for miR-222-221 in the initiation of MM has not been demonstrated. Based on our observation in MM, we hypothesize that this miRNA family may contribute to the initiation and progression of MM. We propose to combine genetic and pharmacological approaches to study the role of miR- 222-221 in MM initiation and its potential to be a therapeutic target. Firstly, we will explore the functioal significance of miR-222-221 and their regulatory network in MM by using the clinical samples and several computational approaches to construct a network of miRNA-mRNA interactions (Aim 1). In parallel, we will functional characterize the role miR-222-221 in MM proliferation, apoptosis and CSC formation by using two preclinical mouse models. One is a conditional transgenic mouse model: Tg:(miR-222-221, AID-Cre mice), which will active miR-222-221 expression in late stage of B cells and plasma cell. Another one is a preclinical human MM cell bearing mouse model (Aim 2). Furthermore, we will functional verify the microRNAs key downstream genes and to evaluate the potential of miR-222-221 as a therapeutic target in vitro and in vivo (Aim 3). Our long-term goal is to understand the role of miRNAs in regulating MM initiation, and to develop new candidate therapies for this malignant disease.